1. Field of the Invention
This invention relates to a process for improving the mechanical properties of metals, and more particularly to a process for producing an aluminum composite having a metal matrix and a reinforcing phase.
2. Description of the Prior Art
An aluminum based composite generally comprises two components --an aluminum alloy matrix and a hard reinforcing second phase. The composite typically exhibits at least one characteristic reflective of each component. For example, an aluminum based metal matrix composite should have the high ductility and fracture toughness of the aluminum matrix and the high elastic modulus of the reinforcing phase.
Aluminum based metal matrix composites containing particulate reinforcements are usually limited to ambient temperature applications because of the large mismatch in higher temperature strength between the aluminum matrix (low strength) and the particle reinforcement (high strength). Another problem with aluminum based metal matrix composites is the difficulty of producing a bond between the matrix and the reinforcing phase. To produce such a bond, it is often times necessary to vacuum hot press the material at temperatures higher than the incipient melting temperature of the matrix. It has been proposed that this technique be avoided by mechanically alloying the matrix with the addition of the particular reinforcement. This procedure, referred to as solid state bonding, permits the reinforcing phase to be bonded to the matrix without heating the material to a temperature above the solidus of the matrix. Prior processes in which aluminum based alloys and/or metal matrix composites are mechanically alloyed by means of solid state bonding are disclosed in U.S. Pat. Nos. 4,722,751, 4,594,222 and 3,591,362.
A major problem with solid state bonding of the particulate into the aluminum alloy matrix is the requirement that a processing aid be added to the powder to allow processing to take place. The processing aid, usually in the form of an organic wax such as stearic acid, must be broken down and the gaseous components removed therefrom by high temperature degassing. Such degassing operations involve prolonged exposure of the material to a temperature of 425.degree. C. or more, degrading the material and limiting application of the material to temperatures below the degassing temperature.